Device for measurement of pulse-to-pulse carrier frequency shift

ABSTRACT

A system for the instantaneous measurement of the carrier frequency in a pulsed radar system, particularly useful in frequency stability analysis of the output of self-excited microwave frequency generators, such as magnetrons, etc. A dispersive delay line, preferably of the type operative in the intermediate frequency domain is employed with appropriate downconversion ahead of the delay line input. A cathode ray display device horizontal sweep is generated synchronously with the magnetron PRF and the output of the dispersive delay line is presented in the appearance of spectral lines on the vertical deflection coordinate thereof. The dispersive delay line inherently provides a frequency time delay conversion so that the cathode ray display device may be calibrated in terms of frequency.

United States Patent 1191 Donahue 1 1 Mar. 27, 1973 [54] DEVICE FORMEASUREMENT OF FOREIGN PATENTS OR APPLICATIONS PULSE-T()-PULSE CARRIER1,028,738 5/1966 Great Britain ..324/82 FREQUENCY SHIFT PrimaryExaminer-Alfred E. Smith [75] Inventor: 9 Donahue, L05 l v Attorney-C.Cornell Remsen, Jr., Walter J. Baum, Callf- Paul W. Hemminger, CharlesL. Johnson, Jr. and [73] Assignee: International Telephones and Tele-Thomas Knsmfferson LY. graph Corporation, New York, N I ABSTRACT [22]Filed: 1970 A system for the instantaneous measurement of the [21] Appl.N0.: 90,577 carrier frequency in a pulsed radar system, particularlyuseful in frequency stability analysis of the output of self-excitedmicrowave frequency generators, such [52] U.S. Cl ..324/82, 324/77 D,324/78 R as magnetrons A dispersive d la line, preferably ..G0ll' of theoperative in the intermediate frequency [58] Field of Search ...324/78R, 82, 77 H, 84, domain is employed with appropriate down conver 332/20;325/134; 33H44' sion ahead of the delay line input. A cathode raydisplay device horizontal sweep is generated Referencesvcltedsynchronously with the magnetron PRP and the output of the dispersivedelay line is presented in the ap- UNITED STATES PATENTS pearance ofspectral lines on the vertical deflection 3,357,021 12/1967 Allen ..324s4 x r in h r f- The di per ive delay line in- 3,416,077 12/1968 Lacy....324/84x herently provides a frequency time delay conversion 2,882,3954/1959 White ..324/78 R so that the cathode ray display device may becalibrated in terms of frequency.

8 Claims, 1 Drawing Figure flrourpur Patented March 27, 1973 INVENTOR.

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6 NR $5 2% i kDORDQQ DEVICE FOR MEASUREMENT OF PULSE-TO- PULSE CARRIERFREQUENCY SHIFT BACKGROUND OF THE INVENTION 1. Field of the InventionThe present invention relates to instantaneous frequency measurement ina pulsed system and, more particularly, to accurate measurements of thattype in pulsed microwave generating systems.

2. Description of the Prior Art In the prior art, specialized devicesare extant for measuring the frequency'of a microwave signal includingthe carrier frequency of a pulsed system. Such pulsed systems arecommonly found in the radar arts.

In the prior art, approximate measurement of frequencies of the G112realm (pulsed or CW) has been accomplished by means of calibratedresonant cavities and similar devices. Methods of frequency measurementsinvolving cycle counting, which are extremely accurate at lowerfrequencies, are not adaptable in the microwave range.

Various attempts have been made to instrument accurate frequencymeasuring devices to measure the carrier frequencies of pulsed radar orsimilar pulsed microwave systems. Many of these are either relativelyinaccurate, or so cumbersome and expensive asto be disadvantageous fromthat point of view.

In one particular prior art instrumentation for measuring the carrierfrequency of a radar pulse, two cascaded hybrids with unequal delaylines inserted between the two output ports of the first hybrid and theinput ports of the second hybrid, provide the frequency sensing element.The power ratio between the two output ports of the second hybrid is arelatively sensitive function of frequency. The measurement of the logof this power ratio is converted to a time difference measurement At bycomparison with an injector signal decaying exponentially. This Atquantity is linearly proportional to frequency over an octave offrequency variation.

The so-called dispersive delay line which is an element in the newcombination of the present invention is, of itself, a prior art devicedeveloped for, and used in, pulse compression FM radar systems. Aparticular type of dispersive delay line, particularly useful in thepreferred instrumentation of the present invention, will be identifiedand described in more detail as this description proceeds.

The manner in which the present invention overcomes the disadvantages ofthe prior art and produces a new and useful structure exhibitingparticular advantages will be understood from the descriptionhereinafter.

SUMMARY OF THE INVENTION The present invention relies on thewell-controlled and highly repeatable characteristics of dispersivedelay lines which were developed for FM pulse compression radar systems.Basically these devices, which operate in the frequency realm of typicalIF frequencies employed in radar systems, are acoustic delay-linedevices excited by input transducers and with transducer output pickoff.Most dispersive delay lines are designed to have a linear change ofdelay in their usable frequency bands, however, linearity, althoughdesirable from a calibration point of view, is not an absolute necessityin connection with the instrumentation of the present invention.

One of the more common dispersive delay lines is the socalled dispersivestrip line which utilizes the first longitudinal mode of a metal strip.Low frequency lines of this type are commonly made from aluminum orsteel and higher frequency lines from steel. The transducers arenormally piezoelectric ceramic.

Another, and preferable type, insofar as the instrumentation of thepresent invention is concerned, is the so-called diffraction dispersivedelay line which uses acoustic diffraction gratings to spatiallydistribute the fourier frequency components of a waveform. Usually suchlines are designed so that this spatial distribution results in a lineardelay versus frequency characteristic, since linearity is particularlydesirable in pulse impres sion applications. Moreover, the bandwidthover which linearity is possible can be on the order of 50 percent, muchlarger than that possible in other types of dispersive delay lines.Also, the center frequency can go as high as MHz. Such a device, havinga frequency versus delay slope of 1 MHz/used, was constructed for thepresent invention.

The reader not specifically familiar with dispersiontype delay lines, isdirected to the technical literature. For example, lines of the typerequired for use in the system of the present invention are described inElectronic News (New Circuits section) in the Monday, Aug. 7, 1967issue. Electronic News is a publication of Fairchild Publishing Co. (NewYork). The developers of the device described were concerned primarilywith improved radar resolution, notwithstanding the fact that itscharacteristics are used for another purpose in the present combination.That reference will also provide' an insight into the availability ofdispersion-type delay lines in various frequency ranges, such as in themicrowave range. Constructed in the microwave range, the down-conversiondescribed herein would be unnecessary.

Basically, the preferred embodiment of the present invention involvesthe sampling, through a coupler and attenuator as necessary, of theoutput of the magnetron or other pulse microwave generator; thedown-conversion (where the VHF type of dispersive delay line is used);application to the dispersive line itself and display of the delayedpulse output against a horizontal sweep on a cathode ray indicatordevice. The said horizontal sweep is synchronously generated withrespect to the magnetron pulses. Thus, the characteristics of thedispersive delay line are employed to convert frequency variations inthe magnetron output into time variations displayable in real time muchlike a spectral line display.

The cathode ray sweep will be linear as long as thefrequency-versus-delay characteristic of the dispersive delay line islinear, but in any event, the said horizontal sweep is readilycalibrated for a direct frequency reading. Other aspects of theinvention and a specific description of a preferred embodiment followhereinafter.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE-drawing illustrates acircuit block diagram for a system in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the FIGURE, aself-excited microwave generator labeled magnetron is illustrated at 1.This device is well known as the most common microwave power oscillatorused in radar systems. A trigger generator 2 is to be understood tocontain the usual timing circuits establishing the pulse repetitionfrequency of the system and also the usual pulse modulator circuitry, sothat the signal on lead 3 is a high power level pulse train whichsupplies the energy, resulting in the generation of high power levelmicrowave oscillations in 1.

The present system is adapted to bench testing, as well as to activesystem monitoring, and accordingly, the RF output from lead 6 wouldnormally pass mostly through the coupler and via the lead 7 either to anantenna or to a dummy load (in the case of bench testing). The coupler 5singles out a relatively small amount of this energy and diverts itthrough a variable attenuator 8 to the mixer 9. This attenuator 8provides attenuation as necessary in order that the level of radiofrequency power supplied to the mixer 9 is appropriate for the type ofmixer used. Normally, such mixers are radar receiving components and, assuch, operate at low power levels, although higher power level mixing iswell known in the art. The stalo" 10 is the system local oscillator andis identified as a stalo because of the desirability of maintaining ahigh order of stability in this component. It will be seen that drift,or instability,

in the local oscillator 10 would produce errors in themeasurement. Thetechnology involved in the construction of such a stable oscillator iswell known in the radar arts, since such devices are normally used inmoving target indicator systems. The mixer output lead 11 isdown-converted to a standard IF band for radar receivers. Normally thisIF band is in the VHF region. The IF driver amplifier 12 functions toboost the power level of the down-converted signal on lead 11.Appropriate levels of power input for a device such as the dispersivedelay line 13 are prescribed by manufacturers of these devices and arewell understood in the prior art.

As will have been previously understood, the dispersive delay line 13 isbasically a frequency sensitive device which converts pulse carrierfrequency changes to changes of pulse envelope delay at its output. Inone particular instrumentation of the present invention, a line having aconversion constant, or sensitivity, of l MHz/usec. was used. Thus, apulse output having a real time position which is variable with respectto the time of occurrence of the magnetron pulse on 6 is delivered tothe IF post delay line amplifier 14. This amplifier compensates forattenuation experienced in the line 13 and builds the signals therefromto an acceptable voltage level to operate a straightforward videodetector 15. Up to 15 the signal will be understood to have been presentin the VHF realm which is the IF frequency for the system. Passing outof 15, the video envelope of the pulses is subjected to a thresholdcircuit 16.

It is a characteristic of a dispersive delay line employed in the mannerillustrated herein, that fourier components of the input pulse thereto,appear at the output. The threshold of the circuit 16 can be said toeliminate the lower amplitude sideband components thus produced, as wellas a certain amount of the in? herent noise present in the signals. Thethreshold circuit 16 output may be standardized in trigger generator 17.Thus, although there may be some variation in the width of signalspassed on by the threshold circuit 16, a standard and preferablynarrower trigger signal passing from 17 to the vertical deflectioncoordinate input of a cathode ray oscillograph 18 produces an easilyread display against the calibration thereon. Returning now to thetrigger generator or system timing circuits in 2, it will be noted thata separate trigger output 4 supplies a trigger pulse of the samefrequency as the signals on 3 to delay trigger generator 19. Thistrigger on 4 may actually precede the corresponding pulses on 3, so thata minimum delay setting in 19 could be made to start the horizontalsweep of the oscillograph 18 ahead of pulses from the dispersive delayline 13 arriving at the vertical deflection coordinate input of theoscillograph 18 via the intervening circuitry including 14 through 17.Thus, nominal magnetron frequency lines will appear near the center ofthe CRT display so that variation on either side. may be presented.Actually then, the adjustment of the delay trigger generator 19 may bethought of as a matter of oscillograph calibration. For clarity,frequency calibration marks 20 are exaggerated somewhat on the FIGURE.If the dispersive delay line sensitivity is 1 MHz/nsec, as aforesaid,and the sweep speed on the oscillograph 18 is 10 usec.,/cm., the overallsystem sensitivity (from magnetron frequency change to horizontal pulsedisplacement on the face of the oscillograph 18) is 10 KHz/cm.

Obviously, the system of the present invention can provide even greatersensitivity if a dispersive line having a slope or sensitivity of lessthan the l MI-Iz/usec. is used. A sensitivity in the delay line of I00KHZ/11366., for example, would increase the sensitivity a full order ofmagnitude to l KHz/cm.

From an understanding of the foregoing, it will be appreciated that thepresent invention is well adapted to the measurement of pulse0to-pulsefrequency shift of a magnetron or other microwave device. Information ofthistype is valuable not only from a bench testing point of view, butalso to predict or monitor magnetron MTI performance capability orsuitability during an actual system monitoring situation.

From an understanding of the foregoing, those skilled in this art willrecognize certain possible modifications and additional applications forthe system of the present invention. One example of an additional usewould be the monitoring of the microwave frequency of the deliberatelyprogrammed frequency hopping radar.

It is to be understood that the present invention is not to be regardedas limited by the illustration or the specific description, these beingintended to be illustrative and typical only.

What is claimed is:

1. Apparatus for measurement of the frequency of the energy output froma pulsed radio frequency generator, comprising:

a mixer and local oscillator of substantially invariant frequencyconnected to receive a portion of said energy output from said pulsedradiofrequency generator to produce mixer output pulses;

a dispersive delay line operative within the frequency domain of saidmixer output pulses and connected to receive said mixer output pulses toproduce corresponding delay line output pulses discretely time delayedby an amount which is a function of the individual frequencies of saidconverter output pulses;

a cathode ray indicating device including means for generating andapplying a sweep in a first deflection coordinate of said indicatingdevice;

synchronizing means for controlling the initiation of said sweep at apredetermined time with respect to said radio-frequency generator outputpulses, said synchronizing means also controlling the initiation of saidenergy pulses from said radio-frequency generator; and

means for applying said delay line output pulses to the seconddeflection coordinate of said indicating means to produce a presentationwhich may be calibrated in frequency along the axis corresponding tosaid first deflection coordinate.

2. A system for instantaneous measurement of the frequency generated bya pulsed radio frequency generator, comprising:

a superheterodyne mixer and a local oscillator of substantially constantfrequency operatively associated therewith;

means for coupling a portion of the pulsed output energy from saidgenerator to said mixer, whereby said mixer produces pulses of energy atintermediate frequency;

a dispersive delay line responsive to said intermediate frequencypulses, for producing output pulses from said delay line which arediscretely delayed with respect to the corresponding inputpulses to saiddelay line as a function of frequency of said radio frequency generatorduring each of said input pulses;

a cathode ray indicating device including a sweep applied to onedeflection coordinate of said indicating device; v

synchronizing means connected for initiating said sweep at apredetermined time related to the occurrence of each of said pulses fromsaid generator; and

means for applying said delay line output to the other deflectioncoordinate of said indicating device.

3. The invention defined in claim 2 including a video detector connectedto receive the output pulses of said delay line thereby to reduce saidpulses to the video frequency realm before application to said otherdeflection coordinate of said indicating device.

4. Apparatus according to claim 3 including a threshold circuit and afollowing trigger generator connected between said video detector andsaid other deflection coordinate of said indicating device, whereby saiddelay line output pulse may be passed only to the extent that theyexceed a predetermined amplitude, and whereby said pulses thus passedare formed into relatively narrow pulses of uniform duration so as to bereadily compared to any frequency calibrations on said indicatingdevice.

5. Apparatus according to claim 3 in which said dispersive delay line isdefined as being of the acoustic diffraction grating type.

6. Apparatus according to claim 5 in which said diffraction grating typeof dispersive delay line is defined as being made in the form of fusedquartz plates.

. Apparatus according to claim 6 including a predeterminable delayabletrigger generating circuit between said' means for synchronizing theinitiation of said sweep and said means for generating and applying saidsweeps to provide calibration of said cathode ray indicating device.

8. Apparatus according to claim 6 including an amplifier preceding saiddispersive delay line to provide an adequate level of drive powertherefor, and an additional amplifier after said dispersive delay lineto com-

1. Apparatus for measurement of the frequency of the energy output from a pulsed radio frequency generator, comprising: a mixer and local oscillator of substantially invariant frequency connected to receive a portion of said energy output from said pulsed radiofrequency generator to produce mixer output pulses; a dispersive delay line operative within the frequency domain of said mixer output pulses and connected to receive said mixer output pulses to produce corresponding delay line output pulses discretely time delayed by an amount which is a function of the individual frequencies of said converter output pulses; a cathode ray indicating device including means for generating and applying a sweep in a first deflection coordinate of said indicating device; synchronizing means for controlling the initiation of said sweep at a predetermined time with respect to said radio-frequency generator output pulses, said synchronizing means also controlling the initiation of said energy pulses from said radio-frequency generator; and means for applying said delay line output pulses to the second deflection coordinate of said indicating means to produce a presentation which may be calibrated in frequency along the axis corresponding to said first deflection coordinate.
 2. A system for instantaneous measurement of the frequency generated by a pulsed radio frequency generator, comprising: a superheterodyne mixer and a local oscillator of substantially constant frequency operatively associated therewith; means for coupling a portion of the pulsed output energy from said generator to said mixer, whereby said mixer produces pulsEs of energy at intermediate frequency; a dispersive delay line responsive to said intermediate frequency pulses, for producing output pulses from said delay line which are discretely delayed with respect to the corresponding input pulses to said delay line as a function of frequency of said radio frequency generator during each of said input pulses; a cathode ray indicating device including a sweep applied to one deflection coordinate of said indicating device; synchronizing means connected for initiating said sweep at a predetermined time related to the occurrence of each of said pulses from said generator; and means for applying said delay line output to the other deflection coordinate of said indicating device.
 3. The invention defined in claim 2 including a video detector connected to receive the output pulses of said delay line thereby to reduce said pulses to the video frequency realm before application to said other deflection coordinate of said indicating device.
 4. Apparatus according to claim 3 including a threshold circuit and a following trigger generator connected between said video detector and said other deflection coordinate of said indicating device, whereby said delay line output pulse may be passed only to the extent that they exceed a predetermined amplitude, and whereby said pulses thus passed are formed into relatively narrow pulses of uniform duration so as to be readily compared to any frequency calibrations on said indicating device.
 5. Apparatus according to claim 3 in which said dispersive delay line is defined as being of the acoustic diffraction grating type.
 6. Apparatus according to claim 5 in which said diffraction grating type of dispersive delay line is defined as being made in the form of fused quartz plates.
 7. Apparatus according to claim 6 including a predeterminable delayable trigger generating circuit between said means for synchronizing the initiation of said sweep and said means for generating and applying said sweeps to provide calibration of said cathode ray indicating device.
 8. Apparatus according to claim 6 including an amplifier preceding said dispersive delay line to provide an adequate level of drive power therefor, and an additional amplifier after said dispersive delay line to compensate for attenuation therein. 